As disclosed in Patent Document 1, there has been a cutting insert configured such that, in a side view, a bottom surface thereof is projected outward so as to form a substantially V shape. Such bottom surface consists of a flat base surface and two inclined surfaces on both sides of such base surface. The two inclined surfaces respectively lead to their corresponding side surfaces, each of which extends so as to form an acute angle with an upper surface of the cutting insert. The cutting insert of Patent Document 1 is mounted on a tool body such that the base surface of the V-shaped bottom surface extends in a direction parallel to a rotational axis of the tool body. At this time, the cutting insert is mounted on a mounting seat of the tool body by screwing a mounting screw into a screw hole via a mounting hole which penetrates the upper surface and the bottom surface. Such configuration allows, from among the two inclined surfaces of the bottom surface, an inclined surface located on the outer periphery side of the tool to receive a supporting force from a surface of the mounting seat of the tool body with which such inclined surface comes into contact, and a force applied to the mounting screw is accordingly reduced. This enhances the fixing property with respect to the mounting seat of the cutting insert.